First trimester, week 3
3 weeks pregnant: A ball of cells finds its home
- Size
- A speck, smaller than a pinhead
- Length
- ≈ 0.2 mm ball of cells
- Weight
- —
- Heart rate
- — not yet
- Due date
- 259 days away 8% of the way
At 3 weeks pregnant, a fertilised egg has become a hollow ball of cells about 0.1–0.2 mm across, travelling down to implant in the lining of the womb.
Your baby this week
- The fertilized egg (zygote) divides again and again as it drifts down the fallopian tube: 2 cells, 4, 8, then a solid ball called a morula.
- By about 5 days after fertilization it becomes a blastocyst, a fluid-filled ball. Its inner cell mass will become the baby, and its outer layer will become the placenta.
- Around 6–10 days after fertilization, the blastocyst burrows into the uterine lining. This is implantation.
Your body this week
- Most people feel nothing yet. A few notice light spotting around implantation.
- After implantation the developing placenta starts making hCG, the hormone that pregnancy tests detect.
- It is usually still too early for a home test to turn positive.
- Typical weight gain so far
- 0.1–0.3 kg
- Your uterus
- About the size of a pear (unchanged to the eye)
Weight gain range is for a pre-pregnancy BMI of 18.5–24.9 (IOM guideline). Your provider may advise differently.
Scans, tests and checkups
-
From week 4Home pregnancy test Self-care
Detects hCG in urine. Most reliable from the first day of a missed period.
-
From week 6First prenatal visit & bloodwork Checkup
Health history, blood pressure, urine, blood type and Rh factor, blood count, and infection screening (hepatitis B, HIV, syphilis, rubella immunity). The UK aims to see you by 10 weeks.
-
From week 6Early / dating ultrasound Ultrasound
Confirms the pregnancy is in the uterus, sees the heartbeat, counts babies and dates the pregnancy. Often part of the first visit or combined with the 12-week scan.
Real size, next to an adult hand
Growth curve
Tip for this week
Avoid alcohol, smoking and recreational drugs from now on. No amount of alcohol is known to be safe in pregnancy.
Myth or fact?
Does the mother determine the baby’s sex?
MythNo. The egg always carries an X chromosome. The sperm carries either an X (girl) or a Y (boy), and that decides the chromosomal sex.
Read the evidenceMore myths around week 3
Questions people ask about week 3
Can a pregnancy test be positive at 3 weeks?
Usually not. hCG only starts to rise after implantation, and most home tests are reliable from the first day of a missed period, which is around week 4.
Is spotting at 3 weeks implantation bleeding?
It can be. Light pink or brown spotting around the time of implantation is fairly common, but it is not a reliable sign of pregnancy. Heavy bleeding or pain should be checked by a clinician.
References
Advice differs between countries, so follow your own care team’s plan. Weights from week 20 are the WHO median, and earlier weights are approximate. How we check this.
All 40 weeks
First trimester Weeks 1–13
From conception to a fully formed, tiny fetus. The most rapid change of the whole pregnancy.
Second trimester Weeks 14–27
Growth, movement and senses. Often the most comfortable stretch for parents.
Third trimester Weeks 28–40
Weight gain, brain and lung maturation, and getting into position for birth.
